The present invention relates to a signal input circuit receiving an external control signal, and more particularly to such a circuit having a signal latch function.
A signal input circuit is provided between an input terminal supplied with an external control signal and an internal circuit utilizing the control signal for a purpose of wave-shaping of the control signal, for example. If the signal input circuit further has a signal latch function, there have been disadvantages that the supply period of the control signal is made short and the supply timing of the control signal with respect to another or other external control signals is not restricted.
A prototype signal input circuit considered by the inventor for the purpose of a signal latch function includes two inverters coupled in cascade and a switch composed of a transistor. The switch is connected between the input terminal and an input end of the first inverter, and an output end of the second inverter is connected to the internal circuit and further to a control end of the first switch.
When the control signal supplied to the input terminal appears at the output end of the second inverter, it is fed back to the first switch, so that the first switch is turned OFF. The input terminal is thereby disconnected from the first inverter. The level at the input end of the first inverter is capactively-held by a stray capacitance coupled thereto, and therefore, the first and second inverters hold their output levels. The supplied control signal is thus latched by the signal input circuit. Accordingly, the control signal can disappear before the operation of the internal circuit finishes.
As a result of investigating this prototype signal input circuit, the inventor has discovered that this circuit is brought into a defective condition of refusing to enter the control signal when a power switch is turned ON to supply a power voltage to the inverters. More specificially, when the power switch is turned OFF to remove the power voltage from the inverter, they do not operate. Accordingly, all electric charges at the respective ends of the inverters and the switch are discharged. If the discharged level at the control terminal of the switch turns it OFF, this OFF-state of the switch is maintained by the feedback loop including the inverters in response to the power switching-on. As a result, the signal input circuit disconnects the input terminal and refuses to enter the control signal.